In the 3rd Generation Partnership Project (“3GPP”) Long Term Evolution (“LTE”) protocol, a user equipment (UE) connects to the core network via nodes (e.g. e-UTRAN NodeB nodes). Typically, the UE connection is maintained over a single node. However, as the UE moves geographically or other network conditions occur, it may be desirable or necessary to transfer the UE connections from the source node (the node to which the UE is currently connected) to an available target node (i.e., perform a handover from the source node to the target node).
The UE connection with the source node is typically encrypted and the cipher requires uplink (“UL”) and downlink (“DL”) counts for each data unit, which comprise sequence numbers and/or a Hyper Frame Numbers (“HFN”). Thus, during a handover the source node must to transfer the UL and DL count values to the target node in order to enable the encryption features of the target node. In the standard 3GPP LTE protocol, this transfer is done in one of two ways depending upon the protocols enabled within the nodes.
If the nodes are configured to operate according to the S1 Application Protocol (“S1AP”), then the transfer of the UL and DL counts occurs via a two step process. First, during an eNB Status Transfer, the UL and DL counts are transmitted from the source node to a Mobile Management Entity (“MME”) connected to the core network. Then, during an MME Status Transfer, the UL and DL counts are transmitted from the MME to the target node.
Alternatively, if the nodes are configured to operate according to the X2 Application Protocol (“X2AP”), then the transfer of the UL and DL counts occurs via a SN Status Transfer. During the SN Status transfer, the UL and DL counts are transmitted directly from the source node to the target node.
After a source node has sent the UL and DL counts to the target node, it must cease scheduling UE data (e.g. data received from the UE or data intended for the UE) so that the UL and DL counts remain accurate when they are received by the target node. It can also occur that a UE connects to a target node (and thus has disconnected from the source node) before the target node has received the UL and DL counts from the source node. In this situation, the UE cannot send or receive any data units via the target node until the target node acquires the UL and DL counts. Thus, during the handover process there can be an undesirable service interruption while the UE waits for the UL and DL counts to be transferred between the source and target nodes.
Furthermore, there may be a finite amount of time between when the UE disconnects from the source node (i.e., the source node stops scheduling UE data) and when the core network is notified that the UE has connected to the target node. During this time period, any DL data units intended for the UE may be sent to the wrong node or lost.